> Doesn't it introduce a weird (stateful!) computation that's hard to reason about?

Yes and no. For example, maybe you want to initialise a temporary directory on the filesystem for writing files to, for some caching mechanism. The pointer to the cache directory (maybe a const x = {}) can exist anywhere, but spinning up the directory itself in the static class block ensures that it's set up once and only once for all instances of the class. So, any instances get to reuse the directory without worrying about checking for initialisation first.

Just another tool in the toolbox, best evaluated on a case-by-case basis